The present invention relates to scanning control systems, and more particularly to improved methods and apparatus for economically and efficiently controlling electron beam scanning in industrial radiation processing to controllably tailor the exposure of the target material to the electron beam.
Electron beam scanning is typically done utilizing an electromagnet which deflects the electron beam back and forth across the target material, usually orthogonally to the direction of travel of both the beam and the target material. The target material is usually moved continuously through a "window" area where the electron beam exits from the accelerator and strikes the target material. The angle of sweep of the electron beam is proportional to the current in the coil of the electromagnet. This current is supplied by an electronic scanning system, so that, in a sense, the scanning electromagnet can be seen as a servomechanism (servo) whose scanning behavior is a result of the scan control signals applied to it by the electronic scanning system.
A typical objective in such industrial electron beam scanning is to achieve uniformity of the electron beam dosage across the target material or product, which means that the electron beam should be scanned linearly across the target material. This is accomplished in many commercial implementations by applying a square wave voltage source to the inductance of the electromagnet such that the full sweep across the target is achieved in the more rapidly changing, nearly linear early portion of the function representing the current of the inductor as a time-based function of the applied square wave voltage.
Not surprisingly, this approach is not always ideal. Scan uniformity (beam dose uniformity) can still be poor (e.g., from parasitic effects) and may lead to product and use variations. Also, certain situations call for non-uniform processing--i.e., for exposing specific regions of the target material to higher beam doses than other regions. For example, a particular sheet material might have its properties significantly enhanced if the edge regions thereof received a significantly greater electron beam treatment than the central region. Both of these situations can be resolved with mechanical masking, but that is often both impractical and wasteful of the electron beam energy. Another solution is to modulate the scan signal from the scanning signal generator so that the servo (scanning magnet) causes the electron beam to exhibit the desired non-linear scanning behavior. However, such electronic circuits are not only challenging to design, but require final power amplifiers whose outputs can be modulated accordingly. This results in unnecessary complexity and expense which is all the more aggravated as the power requirements for the scanning system increase for controlling higher energy beaming systems.
A need therefore remains for improved, uncomplicated, efficient, versatile, and economical methods and apparatus for controlling the scanning or sweeping of the electron beam during exposure of target materials to electron beam radiation. Preferably such improved methods and apparatus will be directly usable and compatible with existing scanner servomechanisms while providing a high degree of easily customizable differential exposure control.